Watlow Series D8 - Manual (Page 122)

Chapter 4: Operation and Setup Series D8 User’s Guide
104 Watlow Anafaze Doc. 0600-3120-2000
With proportional control, when loop 1 is at set point, its out-
put is 0 percent, and the set point of loop 2 is equal to the de-
sired temperature 150º F. If the temperature of the loop 1
drops below 149º F, the deviation results in a proportional out-
put of 10 percent. This results in an increase to the set point
for loop 2 equal to 10 percent of the set point range. In this
case the range is 40º F (190º F - 150º F = 40º F), and 10 percent
of 40º F is 4º F.
So when the temperature at loop 1 drops 1º F, the set point of
loop 2 increases by 4º F until the output of loop 1 is 100 per-
cent and the set point of loop 2 is 190º F. At this point, further
decreases of the loop 1 process variable have no additional af-
fect on loop 2. Figure 4.12 illustrates this relationship.
Figure 4.12 Relationship of Secondary Loop
Set Point to Primary Loop Process
Variable in Cascade Example
Setting Up Ratio Control
Ratio control allows the process variable of one loop (master
loop), multiplied by a ratio, to be the set point of another loop
(ratio loop). You can assign any process variable to determine
the set point of a ratio loop.
By adjusting the ratio control parameters, you can adjust the
influence that the master loop process variable has on the set
point of the ratio loop.
Set Point of the Secondary Loop
(Engineering Units)
Process Variable of Primary Loop (ºF)
Heat Output of Primary Loop
(Percent of Full Scale)
0% 50% 100%
150º F 145º F 140º F
190º F
170º F
150º F